bug: attempt to concatenate local 'qtype' (a nil value)

[initlisk]

Current Behavior

Intermittent error: attempt to concatenate local 'qtype' (a nil value)

Expected Behavior

No response

Error Logs

lua entry thread aborted: runtime error: /usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua:128: attempt to concatenate local 'qtype' (a nil value)
stack traceback:
coroutine 0:
/usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua: in function 'cachelookup'
/usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua:204: in function 'cacheinsert'
/usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua:685: in function 'parseAnswer'
/usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua:736: in function </usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua:719>, context: ngx.timer

Steps to Reproduce

I'm sorry, I don't know how to reproduce this bug. The probability of this bug occurring is very low; it only appears occasionally. By looking at the source code, I couldn't find where the problem might be.

Environment

• APISIX version (run apisix version):2.15.2
• Operating system (run uname -a):5.10.134-13.1.al8.x86_64 SMP x86_64 Linux
• OpenResty / Nginx version (run openresty -V or nginx -V):nginx version: openresty/1.21.4.1
  built by gcc 10.3.1 20210424 (Alpine 10.3.1_git20210424)
  built with OpenSSL 1.1.1g 21 Apr 2020
  TLS SNI support enabled
• etcd version, if relevant (run curl http://127.0.0.1:9090/v1/server_info):
• APISIX Dashboard version, if relevant:
• Plugin runner version, for issues related to plugin runners:
• LuaRocks version, for installation issues (run luarocks --version):/usr/local/bin/luarocks 3.8.0

[initlisk]

I add some log to /usr/local/apisix//deps/share/lua/5.1/resty/dns/client.lua:asyncQuery
`local function asyncQuery(qname, r_opts, try_list)
local key = qname..":"..r_opts.qtype
local item = queue[key]
if item then
--[[
log(DEBUG, PREFIX, "Query async (exists): ", key, " ", fquery(item))
--]]
try_status(try_list, "in progress (async)")
return item -- already in progress, return existing query
end

local nil_qtype = false
if r_opts.qtype == nil then
nil_qtype = true
end

item = {
key = key,
semaphore = semaphore(),
qname = qname,
r_opts = deepcopy(r_opts),
try_list = try_list,
}

if item.r_opts.qtype == nil or nil_qtype then
log(ERR, PREFIX, "item nil qtype in async, ", nil_qtype, " item ropts, ",json(item.r_opts), " key=", key, "try list=", tostring(try_list))
end

queue[key] = item

local ok, err = timer_at(0, executeQuery, item)
if not ok then
queue[key] = nil
log(ERR, PREFIX, "Failed to create a timer: ", err)
return nil, "asyncQuery failed to create timer: "..err
end
--[[
log(DEBUG, PREFIX, "Query async (scheduled): ", key, " ", fquery(item))
--]]
try_status(try_list, "scheduled")

return item
end`

and the error logs is

item nil qtype in async, false item ropts, {} key=***.rds.aliyuncs.com:1try list=["(short)***.rds.aliyuncs.com:(na) - cache-hit/stale","***.rds.aliyuncs.com.test-back.svc.cluster.local:1 - cache-hit/dns server error: 3 name error","***.rds.aliyuncs.com.svc.cluster.local:1 - cache-hit/dns server error: 3 name error","***.rds.aliyuncs.com.cluster.local:1 - cache-hit/dns server error: 3 name error","***.rds.aliyuncs.com:1 - cache-hit/stale"]